I wouldn't exactly say I broke up with her. I was fine trying to continue trying things to change myself. She wanted to have a talk about it and we both agreed it just may be the relationship.
As far as she has the right to, yes, she does. I also have the right to sleep with other women considering we were never married, just in a relationship. The unspoken courtesy of not hurting the other person emotionally doesn't have to necessarily end the exact moment the relationship comes to an end. We still have to see each other every day, talk to each other every day, and cooperate on bills and house chores.
It doesn't make wanting to keep up my end of the bargain with mowing her lawn, or taking my fridge the day I leave without telling her, or not paying my half of the bills when I leave, etc. if she feels nothing is wrong with hitting a one night stand barely weeks after we have ended it while her partner of almost 4 years has to witness the preparation of it. It is understandable for both parties to wait until complete separation. My above responsibilities have just as much legal requirements to uphold as her's to not see anyone else. This is about mutual respect. The only thing that has changed is we don't hang out on the weekends, we have fewer conversations, and we are not physical.
I am sorry about your situation. I truly am. I don't know what the cause of my loss of love for her was. I have some ideas, but it's only speculation and attempts to fix it did not work. I am chalking it up to we were incompatible from the start, and we were too caught up in lust and hopeless romantics to see it. History, love, and the fear of hurting the other is the only reason I think it got so far. I'll miss her. I truly will. And I love her, but I am no longer in love with her.
As far as advice for your situation, I am no expert. I can only offer you my opinion, but with it being such a short relationship, I don't think there will be any coming around. Possibly in one like mine, with added weight behind it like history, but even that may not be enough. I have already begun to move on. I suggest you do the same. It will be easier for you if you do.
Let me know if you would still like to talk.